3.设X1,X2,X3,X4是来自正态总体N(0,2^2)的简单随机样本, X=a(X1-2X2)^2+b(3X2X2)^2+b(3X3-|||--4(x)_(4))}^2, 若统计量X服从x ^2分布,则 ()-|||-(A) =20, b=100 (B) =sqrt (20) =10-|||-(C) =dfrac (1)(sqrt {20)} =dfrac (1)(10) (D) =dfrac (1)(20), =dfrac (1)(100)